The present invention relates to a semiconductor device and a process for producing the same. More particularly, the present invention pertains to a technique which may be effectively employed in a semiconductor device having a conductor layer consisting of a polycrystalline silicon film and either a refractory metal film or a refractory metal silicide film.
There is a growing tendency for semiconductor devices having static random-access memories (hereinafter referred to as "SRAMs") to have an increased packing density. To reduce the area occupied by memory cells in a SRAM, a high-resistance element in each flip-flop circuit is formed from a polycrystalline silicon film. For this purpose, so-called non-doped polycrystalline silicon, in which an impurity such as phosphorus for lowering the resistance is not diffused, is employed.
This high-resistance element is generally formed in a step of forming a second-level conductor layer. One end of the high-resistance element is connected to a supply voltage terminal (e.g., 5[V]) which holds the data "1", and the other end of the element is connected to the gate electrode of a driving MISFET connected to a reference voltage terminal (e.g., 0[V]) which holds the data "0". This gate electrode is generally formed in a step of forming a first-level conductor layer.
The present inventors have examined the above-described conventional technique and have found that the following problems result from the formation of the gate electrode of a MISFET by a combination of a polycrystalline silicon film and a refractory metal silicide film superposed on the upper side of the silicon film for the purpose of increasing the operation speed.
The polycrystalline silicon film which defines the lower layer of the laminated film constituting the gate electrode has an impurity such as phosphorus or arsenic diffused therein for the purpose of lowering the resistance. In a step of forming a polycrystalline silicon film having no impurity doped therein and constituting a high-resistance element, the above-described impurity is diffused into the non-doped polycrystalline silicon film. More specifically, the refractory metal silicide film has a larger diffusion coefficient and a larger outward diffusivity than those of the polycrystalline silicon film. Therefore, such impurity-drawing up effect of the refractory metal silicide or other similar effect causes the impurity in the lower-layer polycrystalline silicon film to be diffused into the refractory metal silicide film, and this causes out-diffusion of the impurity which has been diffused in the silicide film, this impurity then being auto-doped into the polycrystalline silicon film which defines the upper layer. In consequence, variations in resistance are produced between high-resistance elements, and this lowers the yield with respect to the electrical reliability of SRAM. The experiment carried out by the present inventors has confirmed that resistance variations between high-resistance elements lead to an increase in the standby current and, moreover, cause it to vary in a range from 100[.mu.A]to 1[m.mu.]. Thus, the above-described conventional technique involves the disadvantage that the electrical characteristics of SRAM are deteriorated, thus resulting in a defective product.
In addition, the diffusion of an impurity undesirably lowers the resistance of the high-resistance elements, and this leads to an increase in the power consumption of SRAMs.
It should be noted that information concerning SRAMs is contained in "Very Large Scale Integrated Circuit Device Handbook", K.K. SCIENCE FORUM, Nov. 28, 1983, p. 305 to p. 313.